The use of scanning techniques to map surfaces of objects is well known. Prior art FIG. 1 illustrates an object 100 having visible surfaces 101–104. Generally, the visible surfaces 101–103 form a rectangular shape residing on top of a generally planer surface 104.
Projected onto the object 100 is an image, which includes the line 110. In operation, the image of line 110 is received by a viewing device, such as a camera, (not shown) and processed in order to determine the shape of that portion of object 100 where the line 110 resides. By moving the line 110 across the object 100, it is possible to map the entire object 100. Limitations associated with using an image comprising a single line 110 is that a significant amount of time is needed to scan the object 100 to provide an accurate map, and a fixed reference point is needed at either the scanner or the object.
FIG. 2 illustrates a prior art solution to reduce the amount of time taken to scan an object. Specifically, FIG. 2 illustrates an image including lines 121 through 125. By providing multiple lines, it is possible to scan a greater surface area at once, thus allowing for more efficient processing of data associated with the object 100. Limitations of using patterns such as are illustrated in FIG. 2 include the need for a fixed reference point, and that the surface resolution capable of being mapped can be reduced because of the potential for improper processing of data due to overlapping of the discrete portions of the image.
In order to better understand the concept of overlapping, it is helpful to understand the scanning process. Prior art FIG. 3 illustrates the shapes of FIGS. 1 and 2 from a side view such that only surface 102 is visible. For discussion purposes, the projection device (not illustrated) projects a pattern in a direction perpendicular to the surface 101 which forms the top edge of surface 102 in FIG. 3. The point from the center of the projection lens to the surface is referred to as the projection axis, the rotational axis of the projection lens, or the centerline of the projection lens. Likewise, an imaginary line from a center point of the viewing device (not shown) is refereed to as the view axis, the rotational axis of the view device, or the centerline of the view device, extends in the direction which the viewing device is oriented.
The physical relationship of the projection axis and the view axis with respect to each other is generally known. In the specific illustration of FIG. 3, the projection axis and the view axis reside in a common plane. The relationship between the projection system and the view system is physically calibrated, such that the relationship between the projector, and the view device is known. Note the term “point of reference” is to describe the reference from which a third person, such as the reader, is viewing an image. For example, for FIG. 2, the point of reference is above and to the side of the point that is formed by surfaces 101, 102, and 103.
FIG. 4 illustrates the object 100 with the image of FIG. 2 projected upon it where the point of reference is equal to the projection angle. When the point of reference is equal to the projection angle, no discontinuities appear in the projected image. In other words, the lines 121–125 appear to be straight lines upon the object 100. However, where the point of reference is equal to the projection axis, no useful data for mapping objects is obtained, because the lines appear to be undistorted.
FIG. 5 illustrates the object 100 from a point of reference equal to the view angle of FIG. 2. In FIG. 5, the surfaces 104, 103 and 101 are visible because the view axis is substantially perpendicular to the line formed by surfaces 101 and 103, and is to the right of the plane formed by surface 102, see FIG. 2, which is therefore not illustrated in FIG. 5. Because of the angle at which the image is being viewed, or received by the viewing device, the lines 121 and 122 appear to be a single continuous straight line. Likewise, line pairs 122 and 123, and 123 and 124, coincide to give the impression that they are single continuous lines. Because line 125 is projected upon a single level surface elevation, surface 104, line 125 is a continuous single line.
When the pattern of FIG. 5 is received by a processing device to perform a mapping function, the line pairs 121 and 122, 122 and 123, and 123 and 124, will be improperly interpreted as single lines. As a result, the two-tiered object illustrated in FIG. 2 may actually be mapped as a single level surface, or otherwise inaccurately displayed because the processing steps can not distinguish between the line pairs.
FIG. 6 illustrates a prior art solution for overcoming the problem described in FIG. 5. Specifically, FIG. 6 illustrates the shape 100 having an image projected upon it whereby a plurality of lines having different line widths, or thickness, are used. FIG. 7 illustrates the pattern of FIG. 6 from the same point of reference as that of FIG. 5.
As illustrated in FIG. 7, it is now possible for a processing element analyzing the received data to distinguish between the previously indistinguishable line pairs. Referring to FIG. 7, line 421 is still lined up with line 422 to form what appears to be a continuous line. However, because line 421 and line 425 have different thickness, it is now possible for an analysis of the image to determine the correct identity of the specific line segments. In other words, the analysis of the received image can now determine that line 422 projected on surface 104, and line 422 projected on surface 101 are actually a common line. Utilizing this information, the analysis of the received image can determine that a step type feature occurs on the object being scanned, resulting in the incongruity between the two segments of line 422.
While the use of varying line thickness, as illustrated in FIG. 7, assists identifying line segments, objects that have varying features of the type illustrated can still result in errors during the analysis of the received image.
FIG. 8 illustrates from a side point of reference a structure having a surface 710 with sharply varying features. The surface 710 is illustrated to be substantially perpendicular to the point of reference of FIG. 8. In addition, the object 700 has side surfaces 713 and 715, and top surfaces 711 and 712. From the point of reference of FIG. 8, the actual surfaces 711, 712, 713 and 715 are not viewed, only their edges are represented. The surface 711 is a relatively steep sloped surface, while the surface 712 is a relatively gentle sloped surface.
Further illustrated in FIG. 8 are three projected lines 721 through 723 having various widths. A first line 721 has a width of four. A second projected line 722 has a width of one. A third projected line 723 has a width of eight.
The line 721, having a width of four, is projected onto a relatively flat surface 714. Because of the angle between the projection axis and the view axis, the actual line 721 width viewed at the flat surface 714 is approximately two. If the lines 722 and 723 where also projected upon the relatively flat surface 714 their respected widths would vary by approximately the same proportion amount as that of 721, such that the thickness can be detected during the analysis steps of mapping the surface. However, because line 722 is projected onto the angled surface 711, the perspective from the viewing device along the viewing axis is such that the line 722 has a viewed width of two.
Line 722 appears to have a width of two because the steep angle of the surface 710 allows for a greater portion of the projected line 722 to be projected onto a greater area of the surface 711. It is this greater area of the surface 722 that is viewed to give the perception that the projected line 722 has a thickness of two.
In a manner opposite to how line 722 is affected by surface 711, line 723 is affected by surface 712 to give the perception that the projected line 723 having an actual width of eight, has a width of two. This occurs because the angle of the surface 712, relative to the viewing device allows the surface area with the projected line 723 to appear to have a width of two. The result of this phenomenon is further illustrated in FIG. 9.
FIG. 9 illustrates the shape 700 of FIG. 8 from the point of reference of the view axis. From the point of reference of the view axis, the lines 721–723 are projected onto the surface 714 in such a manner that the difference between the line thickness can be readily determined. Therefore, when an analysis of the surface area 714 occurs, the lines are readily discernable based upon the viewed image. However, when an analysis includes the surfaces 711 and 712, the line 722 can be erroneously identified as being line 721 because not only are the widths the same, but line 722 on surface 711 lines up with line 721 on surface 714. Likewise, the line 723, having a projected width of eight, has a viewed width of two. Therefore, during the analysis of the received images, it may not be possible to distinguish between lines 721, 722, and 723 on surfaces 711 and 712. The inability to distinguish between such lines can result in an erroneous analysis of the surfaces.
One proposed method of scanning, disclosed in foreign patent DE 198 21 611.4, used a pattern that had rows of black and white triangles and squares running parallel to a plane of triangulation. The rows used measuring features that include a digital encrypted pattern. However, when a surface being scanned causes shadowing and/or undercuts, a break in the sequence can result due to a portion of the pattern being hidden. Furthermore, the disclosed encrypted pattern is such that breaks in the sequence can result in the inability to decode the pattern, since it may not be possible to know which portion of the pattern is missing. A further limitation of the type of encoding described is that distortion can cause one encoding feature to look like another. For example, a triangle can be made to look like a square.
Therefore, a method and apparatus capable of overcoming the problems associated with the prior art mapping of objects would be advantageous.